The present invention relates to a spatial light modulator used in an optical arithmetic device or a video projector and, more particularly, to a spatial light modulator providing improved contrast.
Spatial light modulators (SLM) can transform incoherent light into coherent light, or vice versa. Application of spatial light modulators to parallel processing of data and to direct arithmical processing of image has been discussed. If the intensity of light can be amplified, spatial light modulators can be applied in display systems such as video projectors.
Various spatial light modulators of this kind are known as discussed in APPLIED PHYSICS LETTERS, Vol. 22, No. 3, Feb. 1, 1973, pp. 90-92, Preprint for the 50th Meeting (Autumn 1989) of the Japan Society of Applied Physics, 28P-ZD-5-7, and Japanese Patent Laid-Open No. 93519/1990.
One of such known spatial light modulators is shwon in FIG. 1, where a dielectric mirror 12 and an insulating light-blocking layer 14 are laminated on the incident side of a photomodulation layer 10, on which a writing light beam impinges. Further on the blocking layer 14 a photoconductive layer 16 is formed. A transparent electrode 18 and a glass substrate 20 are laminated in the order over the photoconductive layer 16.
On the other hand, a transparent electrode 22 and a glass substrate 24 are stacked on the side of the photomodulation layer 10 on which a reading light impinges. An appropriate driving power supply 26 is connected between the transparent electrode 18 and another transparent electrode 22.
The aforementioned photomodulation layer 10 consists of a film of a liquid crystal or a high polymer in which liquid crystalline molecules are dispersed. The dielectric mirror 12 consists either of a lamination of a titanium dioxide (TiO.sub.2) film and a silicon dioxide (SiO.sub.2) film or of a lamination of a silicon (Si) film and a SiO.sub.2 film The light-blocking layer 14 is made from silicon (Si), germanium (Ge), boron (B), or other materials.
The photoconductive layer 16 is fabricated from hydrogenated amorphous silicon (a-Si:H), hydrogenated amorphous silicon carbide (a-SiC:H), hydrogenated amorphous silicon germanium (a-SiGe:H), crystallized bismuth silicon oxide (Bi.sub.12 SiO.sub.20), or cadmium sulfide (CdS). Of these materials, a-Si:H, a-SiC:H, and a-SiGe:H are deposited as thin layers by plasma-assisted chemical vapor deposition (PCVD) or other process. CdS is deposited as a thin film by vacuum evaporation. The transparent electrodes 18 and 22 are made from indium-tin oxide (ITO) or stannic oxide (SnO.sub.2).
The operation of the spatial light modulator (SLM) built as described above is now described briefly. The writing light containing desired information passes through the glass substrate 20 and the transparent electrode 18 of the device and enters the photoconductive layer 16 as indicated by the arrow F1. Electron-hole pairs are generated in the photoconductive layer 16 according to the intensity distribution of the beam of writing light. Then, these pairs are separated to form an image of electric charge corresponding to the distribution of the intensity of the writing light.
On the other hand, the reading light hits the photomodulation layer 10 as indicated by the arrow F2. However, an electric field generated by the charge image formed in the photoconductive layer 16 is applied to this photomodulation layer 10. It follows that optical modulation is achieved according to the strength of the field, thus to the intensity distribution of the writing light. The reading light modulated by the photomodulation layer 10 is reflected by the dielectric mirror 12 and leaves the device as indicated by the arrow F3.
The light-blocking layer 14 prevents the reading light passed through the dielectric mirror 12 from reaching the photoconductive layer 16; otherwise the image of electric charge would be disturbed, leading to a reduction in the contrast of the image read out.
Important figures of merit of spatial light modulators include the availability of the light and contrast ratio, as well as resolution and response. Among others, contrast ratio materially affects the performance of the device and depends much on the characteristics of the photomodulation layer 10. If contrast ratio is simply defined as the ratio of the brightness in the bright portions of the image to the brightness in the dark portions, then it follows that the contrast ratio is improved by making the bright portions as bright as possible and the dark portions as dark as possible. Where the photomodulation layer 10 exhibits good characteristics, the light intensity in the dark portions especially greatly affects the contrast ratio.
In a spatial light modulator whose photomodulation layer 10 comprises a liquid crystalline material of twisted nematic structure, the reading light is polarized light, and that component of the light reflected from the surface which has the same angle of polarization as the reading light is cut. Therefore, the availability of the light deteriorates slightly, but the contrast is hardly affected by the reading light.
However, where the photomodulation layer 10 utilizes a scattering type material such as a high polymer in which a liquid crystalline material is dispersed, the reading light is affected noticeably by the reflection at the surface. In this case, a high contrast ratio cannot be obtained.
FIG. 2 particularly shows the reflection of the reading light F2 used for the conventional structure shown in FIG. 1 in the vicinities of the surface of the SLM. Principal reflections include reflection at the surface of the glass substrate 24 (indicated by the arrow F4), reflection at the interface between the glass substrate 24 and the transparent electrode 22 due to different refractive indices (indicated by the arrow F5), and reflection at the interface between the transparent electrode 22 and the light modulator 10 due to the difference in refractive index (indicated by the arrow F6). Of these reflections, the reflection at the surface of the glass substrate 24 (indicated by the arrow F4) is generally of the order of 4%. This reflection can be suppressed to below 0.5% by forming an antireflection coating.
However, there is a possibility that the reflectivity at each surface of the transparent electrode 22 is considerably high because the refractive index of the transparent electrode 22 is as high as about 2. On the other hand, the refractive index of the glass substrate 24 is typically 1.48.about.1.52 and the same of the light modulator 10 made of liquid crystalline dispersed polymer mentioned previously, is typically 1.47.about.1.48. As an example, it is assumed that the spatial light modulator is fabricated by using the scattering type liquid crystal material which permits the dielectric mirror 12 to reflect 100% and 1% of the incident light at its maxium and minimum, respectively. If no reflection occurs at the surface indicated by the arrows F4, F5, and F6, then the maximum contrast ratio is 100:1. However, if reflection of 5% takes place at this surface, then the light intensity in the dark portions of the image increases to at least 6% (1%+5%). In this way, the contrast ratio is given by EQU 100:6=16.7:1
If measures are taken to prevent the reflection at the surface of the glass substrate 24, the surface reflection can be reduced by about 3.5%. In this case, the contrast ratio is improved up to 100:2.5=40:1, because the light intensity in the dark portions of the image is 1%+(5-3.5)%=2.5%. However, where the light modulator is applied to an image display, it is necessary to improve the contrast ratio by a factor of 2 or more. Hence, it is necessary to reduce the reflection at the surface further.
Such effect of light reflection occurs also on the writing side of the spatial light modulator. In particular, the reflectivity for the writing light increases, depending on the thickness of the film of the transparent electrode 18 on the writing side. This deteriotes the contrast ratio of the image or produces flare, thus leading to a reduction in there solution. In this way, undesirable phenomena take place.